What Causes Autism? The Science Behind ASD

The Genetics of Autism

The strongest evidence about autism causation comes from genetic research. Twin studies, comparing identical twins (who share nearly all DNA) with fraternal twins (who share about half), consistently show that when one identical twin is autistic, the other twin is autistic at much higher rates than seen in fraternal twins. This heritability signal indicates that genetic factors account for a large proportion of autism risk, with estimates typically ranging from 60% to 90%.

However, autism genetics is not simple. No single gene causes autism. Hundreds of genes have been identified as contributing to autism risk, and the interactions between them are complex. Some cases of autism are associated with specific rare genetic mutations or chromosomal variations (such as those seen in Fragile X syndrome, Rett syndrome, or 22q11.2 deletion syndrome), but these account for only a minority of autism cases. The majority of autism appears to arise from the combined effect of many common genetic variants, each contributing a small amount of risk.

Research into the genetic architecture of autism is ongoing. Large genome-wide association studies (GWAS) continue to identify new genetic associations. This research may eventually contribute to earlier identification, but it is important to note that autism genetics research is primarily aimed at understanding, not at developing prenatal screening or elimination of autistic traits.

Environmental Risk Factors

While genetics is the dominant factor, researchers have also identified several environmental factors associated with modest increases in autism risk. These are correlational associations, they do not prove that these factors cause autism, and most children exposed to these factors are not autistic.

Advanced parental age: Studies have found modest associations between older parental age, both maternal and paternal, and autism risk in offspring. The proposed mechanism is that de novo (new, not inherited) genetic mutations become more common with age in both egg and sperm cells.

Prenatal exposures: Some research suggests that prenatal exposure to certain medications, particularly valproate (used to treat epilepsy and bipolar disorder) , is associated with increased autism risk. Mothers taking valproate during pregnancy should discuss this with their prescribing physician. Prenatal infection, significant maternal inflammation, and preterm birth have also been studied as potential risk factors, with modest associations found in some research.

None of these environmental factors constitute a clear, modifiable "cause" that parents could have avoided. Autism is not caused by parenting practices, diet, screen time, or most of the factors commonly speculated about in online discussions.

Why Autism Diagnoses Have Increased

Autism is diagnosed more frequently today than it was 30 years ago. Some people interpret this as evidence of an epidemic caused by environmental factors. The scientific consensus, however, points to a different explanation: the increase in diagnoses reflects changes in how autism is diagnosed and recognized, not a true increase in the underlying prevalence of autism.

Key factors driving increased diagnosis rates include: broadened diagnostic criteria (particularly the DSM-5 in 2013, which folded Asperger Syndrome and PDD-NOS into the single ASD category); greater public and professional awareness of autism across the spectrum; recognition that autism presents differently in girls and women, who were previously frequently missed; improved access to assessment in many regions; reduced stigma making families more likely to seek assessment; and the recognition that many adults who went undiagnosed in childhood are now receiving assessments.

Understanding autism causes accurately is important for families navigating diagnosis and for those who are advocating for better services. To learn about the Ontario autism support landscape, see our guides to autism spectrum disorder, OAP eligibility, and autism signs at every age.

Frequently Asked Questions